Non-naturally balanced feed system for an injection molding apparatus

ABSTRACT

A low constant pressure injection molding machine forms molded parts by injecting molten thermoplastic material into a mold cavity at low constant pressures of 6,000 psi and lower. As a result, the low constant pressure injection molding machine includes a mold formed of easily machineable material that is less costly and faster to manufacture than typical injection molds.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to apparatuses and methods for injection molding and, more particularly, to apparatuses and methods for producing injection molded parts at low constant pressure.

BACKGROUND OF THE INVENTION

Injection molding is a technology commonly used for high-volume manufacturing of parts made of meltable material, most commonly of parts made of thermoplastic polymers. During a repetitive injection molding process, a plastic resin, most often in the form of small beads or pellets, is introduced to an injection molding machine that melts the resin beads under heat, pressure, and shear. The now molten resin is forcefully injected into a mold cavity having a particular cavity shape. The injected plastic is held under pressure in the mold cavity, cooled, and then removed as a solidified part having a shape that essentially duplicates the cavity shape of the mold. The mold itself may have a single cavity or multiple cavities. Each cavity may be connected to a flow channel by a gate, which directs the flow of the molten resin into the cavity. A molded part may have one or more gates. It is common for large parts to have two, three, or more gates to reduce the flow distance the polymer must travel to fill the molded part. The one or multiple gates per cavity may be located anywhere on the part geometry, and possess any cross-section shape such as being essentially circular or be shaped with an aspect ratio of 1.1 or greater. Thus, a typical injection molding procedure comprises four basic operations: (1) heating the plastic in the injection molding machine to allow it to flow under pressure; (2) injecting the melted plastic into a mold cavity or cavities defined between two mold halves that have been closed; (3) allowing the plastic to cool and harden in the cavity or cavities while under pressure; and (4) opening the mold halves to cause the part to be ejected from the mold.

The molten plastic resin is injected into the mold cavity and the plastic resin is forcibly pushed through the cavity by the injection molding machine until the plastic resin reaches the location in the cavity furthest from the gate. The resulting length and wall thickness of the part is a result of the shape of the mold cavity.

While it may be desirous to reduce the wall thickness of injected molded parts to reduce the plastic content, and thus cost, of the final part; reducing wall thickness using a conventional injection molding process can be an expensive and a non-trivial task, particularly when designing for wall thicknesses less than 15, 10, 3, and 1.0 millimeter. As a liquid plastic resin is introduced into an injection mold in a conventional injection molding process, the material adjacent to the walls of the cavity immediately begins to “freeze,” or solidify and cure. As the material flows through the mold, a boundary layer of material is formed against the sides of the mold. As the mold continues to fill, the boundary layer continues to thicken, eventually closing off the path of material flow and preventing additional material from flowing into the mold. The plastic resin freezing on the walls of the mold is exacerbated when the molds are cooled, a technique used to reduce the cycle time of each part and increase machine throughput.

There may also be a desire to design a part and the corresponding mold such that the liquid plastic resin flows from areas having the thickest wall thickness towards areas having the thinnest wall thickness. Increasing thickness in certain regions of the mold can ensure that sufficient material flows into areas where strength and thickness is needed. This “thick-to-thin” flow path requirement can make for inefficient use of plastic and result in higher part cost for injection molded part manufacturers because additional material must be molded into parts at locations where the material is unnecessary.

One method to decrease the wall thickness of a part is to increase the pressure of the liquid plastic resin as it is introduced into the mold. By increasing the pressure, the molding machine can continue to force liquid material into the mold before the flow path has closed off. Increasing the pressure, however, has both cost and performance downsides. As the pressure required to mold the component increases, the molding equipment must be strong enough to withstand the additional pressure, which generally equates to being more expensive. A manufacturer may have to purchase new equipment to accommodate these increased pressures. Thus, a decrease in the wall thickness of a given part can result in significant capital expenses to accomplish the manufacturing via conventional injection molding techniques.

Additionally, when the liquid plastic material flows into the injection mold and rapidly freezes, the polymer chains retain the high levels of stress that were present when the polymer was in liquid form. The frozen polymer molecules retain higher levels of flow induced orientation when molecular orientation is locked in the part, resulting in a frozen-in stressed state. These “molded-in” stresses can lead to parts that warp or sink following molding, have reduced mechanical properties, and have reduced resistance to chemical exposure. The reduced mechanical properties are particularly important to control and/or minimize for injection molded parts such as thinwall tubs, living hinge parts, and closures.

In an effort to avoid some of the drawbacks mentioned above, many conventional injection molding operations use shear-thinning plastic material to improve flow of the plastic material into the mold cavity. As the shear-thinning plastic material is injected into the mold cavity, shear forces generated between the plastic material and the mold cavity walls tend to reduce viscosity of the plastic material, thereby allowing the plastic material to flow more freely and easily into the mold cavity. As a result, it is possible to fill thinwall parts fast enough to avoid the material freezing off before the mold is completely filled.

Reduction in viscosity is directly related to the magnitude of shear forces generated between the plastic material and the feed system, and between the plastic material and the mold cavity wall. Thus, manufacturers of these shear-thinning materials and operators of injection molding systems have been driving injection molding pressures higher in an effort to increase shear, thus reducing viscosity. Typically, injection molding systems inject the plastic material in to the mold cavity at melt pressures of 15,000 psi or more. Manufacturers of shear-thinning plastic material teach injection molding operators to inject the plastic material into the mold cavities above a minimum melt pressure. For example, polypropylene resin is typically processed at pressures greater than 6,000 psi (the recommended range from the polypropylene resin manufacturers, is typically from greater than 6,000 psi to about 15,000 psi. Resin manufacturers recommend not to exceed the top end of the range. Press manufacturers and processing engineers typically recommend processing shear thinning polymers at the top end of the range, or significantly higher, to achieve maximum potential shear thinning, which is typically greater than 15,000 psi, to extract maximum thinning and better flow properties from the plastic material. Shear thinning thermoplastic polymers generally are processed in the range of over 6,000 psi to about 30,000 psi.

The molds used in injection molding machines must be capable of withstanding these high melt pressures. Moreover, the material forming the mold must have a fatigue limit that can withstand the maximum cyclic stress for the total number of cycles a mold is expected to run over the course of its lifetime. As a result, mold manufacturers typically form the mold from materials having high hardness, typically greater than 30 Rc, and more typically greater than 50 Rc. These high hardness materials are durable and equipped to withstand the high clamping pressures required to keep mold components pressed against one another during the plastic injection process. These high hardness materials are also better able to resist wear from the repeated contact between molding surfaces and polymer flow.

High production injection molding machines (i.e., class 101 and class 102 molding machines) that produce thinwalled consumer products exclusively use molds having a majority of the mold made from the high hardness materials. High production injection molding machines typically produce 500,000 cycles per year or more. Industrial quality production molds must be designed to withstand at least 500,000 cycles per year, preferably more than 1,000,000 cycles per year, more preferably more than 5,000,000 cycles per year, and even more preferably more than 10,000,000 cycles per year. These machines have multi cavity molds and complex cooling systems to increase production rates. The high hardness materials are more capable of withstanding the repeated high pressure clamping operations than lower hardness materials. However, high hardness materials, such as most tool steels, have relatively low thermal conductivities, generally less than 20 BTU/HR FT ° F., which leads to long cooling times as heat is transferred through from the molten plastic material through the high hardness material.

In an effort to reduce cycle times, typical high production injection molding machines having molds made of high hardness materials include relatively complex internal cooling systems that circulate cooling fluid within the mold. These cooling systems accelerate cooling of the molded parts, thus allowing the machine to complete more cycles in a given amount of time, which increases production rates and thus the total amount of molded parts produced. In some class 101, more than 1 or 2 million cycles per year may be run, these molds are sometimes referred to as “ultra high productivity molds.” Class 101 molds that run in 400 ton or larger presses are sometimes referred to as “400 class” molds within the industry.

Another drawback to using high hardness materials for the molds is that high hardness materials, such as tool steels, generally are fairly difficult to machine. As a result, known high throughput injection molds require extensive machining time and expensive machining equipment to form, and expensive and time consuming post-machining steps to relieve stresses and optimize material hardness.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplary in nature and not intended to limit the subject matter defined by the claims. The following detailed description of the illustrative embodiments can be understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:

FIG. 1 illustrates a schematic view of an injection molding machine constructed according to the disclosure;

FIG. 2 illustrates one embodiment of a thin-walled part formed in the injection molding machine of FIG. 1;

FIG. 3 is a cavity pressure vs. time graph for the injection molding machine of FIG. 1;

FIG. 4 is a cross-sectional view of one embodiment of a mold of the injection molding machine of FIG. 1;

FIG. 5 is a perspective view of a feed system; and

FIGS. 6A and 6B are top and front views of a naturally balanced feed system.

FIGS. 7A and 7B are top and front views of another naturally balanced feed system.

FIG. 8 is a top view of an artificially balanced feed system that may be used in the injection molding machine of FIG. 1.

FIGS. 9A and 9B are top views of non-balanced feed systems that may be used in the injection molding machine of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention generally relate to systems, machines, products, and methods of producing products by injection molding and more specifically to systems, products, and methods of producing products by low constant pressure injection molding.

The term “low pressure” as used herein with respect to melt pressure of a thermoplastic material, means melt pressures in a vicinity of a nozzle of an injection molding machine of approximately 6000 psi and lower.

The term “substantially constant pressure” as used herein with respect to a melt pressure of a thermoplastic material, means that deviations from a baseline melt pressure do not produce meaningful changes in physical properties of the thermoplastic material. For example, “substantially constant pressure” includes, but is not limited to, pressure variations for which viscosity of the melted thermoplastic material does not meaningfully change. The term “substantially constant” in this respect includes deviations of up to approximately 30% from a baseline melt pressure. For example, the term “a substantially constant pressure of approximately 4600 psi” includes pressure fluctuations within the range of about 6000 psi (30% above 4600 psi) to about 3200 psi (30% below 4600 psi). A melt pressure is considered substantially constant as long as the melt pressure fluctuates no more than 30% from the recited pressure.

Balance of fill is a term used to define the flow balance of a given plastic as it is dynamically distributed throughout the injection mold system. The plastic distribution system encompasses the hot or cold runner system, as well as the cavity. An injection mold system can be one that is naturally balanced, artificially balanced or unbalanced.

Balance of fill is measured by the cavity-to-cavity weight differences and provides an indication of the performance of the hot or cold runner system. Where a good performing runner system is measured by how evenly the polymer fills each individual cavity, and where a perfect runner system would fill each cavity at exactly the same time. In conventional injection molding, it is critical to have the flows balanced to each cavity or the part-to-part variation may be large and process capability may not be achievable. A mold with an acceptable flow balance will reduce variation in part weight, dimensional variation, and shrink rates across every cavity in the mold.

The level of imbalance is based on a comparison of the weight of all individual parts in the mold as measured against the mean weight of all cavities in the mold. The measurements are taken when the first cavity to fill reaches 100% of fill, the injection process is stopped, and all parts are weighed to compare the range of weights relative to each other. Cavity to cavity imbalance is calculated against the mean part weight. The formula below shows the calculation for calculating the imbalance using this method:

${\%\mspace{14mu}{Im}\;{balance}} = \frac{\left( {W_{a} - W_{n}} \right)*100}{W_{a}}$ Where Wa=Average Weight of all the cavities Wn=Weight of cavity n, where n=cavity number An acceptable fill balance is one where all part weights are generally typical generally within +/−10% of the mean, more desirably +/−5%, and ideally +/−1%.

Hot runner design is a critical element for a robust conventional injection molding process and mold design. Inferior hot runner designs can lead to excessive part defects and added part costs. In order to achieve natural balance with a conventional molding process, the material must flow through identical runner geometries from the machine nozzle to each of the gates. This means not just the same flow distance, but the same bore diameters and the same number of turns along the flow path. The design of these balanced hot runner systems are typically based on common design principles; 1). The pressure drop across the hot runner system is preferably less than 6,000 psi, 2.) there is at preferably no more than three times the volume of molten plastic material contained in the hot runner system as the volume contained in the sum of all of the mold cavities, and 3) the geometry of hot runner flow branches is optimized to eliminate molten plastic dead spots, or areas where molten plastic becomes trapped and unable to flow through the system. These principles often result in oversized bore diameters in order to achieve the prescribed pressure drop requirements, and can lead to higher ratios of runner volume to part volume than may be desired due to this pressure limitation.

Constant pressure injection molding allows hot runner systems to be filled at pressure substantially lower than conventional injection molding, which enables the use of smaller and more consistent bore diameters throughout the hot runner system while achieving pressure drops of less than 6000 psi. In the case of low pressure molding, such as where machine nozzle pressures are lower than about 10,000 psi, or even more preferably less than about 6,000 psi, it is possible to achieve pressure drop across the manifold of less than about 3,000 psi, or more preferably less than 2,000 psi, or even more preferably less than about 1,000 psi. This allows for the ability to achieve substantially lower ratios of runner volume to mold cavity volume, provides for less material dead spots at intersection of runner branch diameters, and allows for improved flow balance. Furthermore, the reduced runner volume requires less heat in the runner system to maintain the molten polymer at the desired processing temperature. In the case of artificially and naturally balanced feed systems, constant pressure processing maintains very good balance of fill, such as a mold balance of 90% or even more preferably 95% or higher, even when material viscosity differences are introduced as a result of material batch variations, melt temperature variations, or mold temperature variations. It is understood, that when considering bore diameter a channel of any cross sectional area and corresponding cross sectional profile can be used to form the runner channels. However, cylindrical runner channels are commonly used to facilitate ease of channel fabrication and minimize frictional forces on molten polymer.

Referring to the figures in detail, FIG. 1 illustrates an exemplary low constant pressure injection molding apparatus 10 for producing thin-walled parts in high volumes (e.g., a class 101 or 102 injection mold, or an “ultra high productivity mold”). The injection molding apparatus 10 generally includes an injection system 12 and a clamping system 14. A thermoplastic material may be introduced to the injection system 12 in the form of thermoplastic pellets 16. The thermoplastic pellets 16 may be placed into a hopper 18, which feeds the thermoplastic pellets 16 into a heated barrel 20 of the injection system 12. The thermoplastic pellets 16, after being fed into the heated barrel 20, may be driven to the end of the heated barrel 20 by a reciprocating screw 22. The heating of the heated barrel 20 and the compression of the thermoplastic pellets 16 by the reciprocating screw 22 causes the thermoplastic pellets 16 to melt, forming a molten thermoplastic material 24. The molten thermoplastic material is typically processed at a temperature of about 130° C. to about 410° C.

The reciprocating screw 22 forces the molten thermoplastic material 24, toward a nozzle 26 to form a shot of thermoplastic material, which will be injected into a mold cavity 32 of a mold 28. The molten thermoplastic material 24 may be injected through a gate 30, which directs the flow of the molten thermoplastic material 24 to the mold cavity 32. The mold cavity 32 is formed between first and second mold parts 25, 27 of the mold 28 and the first and second mold parts 25, 27 are held together under pressure by a press or clamping unit 34. The press or clamping unit 34 applies a clamping force in the range of approximately 1000 psi to approximately 6000 psi during the molding process to hold the first and second mold parts 25, 27 together while the molten thermoplastic material 24 is injected into the mold cavity 32. To support these clamping forces, the clamping system 14 may include a mold frame and a mold base, the mold frame and the mold base being formed from a material having a surface hardness of more than about 165 BHN and preferably less than 260 BHN, although materials having surface hardness BHN values of greater than 260 may be used as long as the material is easily machineable, as discussed further below.

Once the shot of molten thermoplastic material 24 is injected into the mold cavity 32, the reciprocating screw 22 stops traveling forward. The molten thermoplastic material 24 takes the form of the mold cavity 32 and the molten thermoplastic material 24 cools inside the mold 28 until the thermoplastic material 24 solidifies. Once the thermoplastic material 24 has solidified, the press 34 releases the first and second mold parts 25, 27, the first and second mold parts 25, 27 are separated from one another, and the finished part may be ejected from the mold 28. The mold 28 may include a plurality of mold cavities 32 to increase overall production rates. The shapes of the cavities of the plurality of mold cavities may be identical, similar or different from each other. (The latter is the family of mold cavities).

A controller 50 is communicatively connected with a sensor 52 and a screw control 36. The controller 50 may include a microprocessor, a memory, and one or more communication links The controller 50 may be connected to the sensor 52 and the screw control 36 via wired connections 54, 56, respectively. In other embodiments, the controller 50 may be connected to the sensor 52 and screw control 56 via a wireless connection, a mechanical connection, a hydraulic connection, a pneumatic connection, or any other type of communication connection known to those having ordinary skill in the art that will allow the controller 50 to communicate with both the sensor 52 and the screw control 36.

In the embodiment of FIG. 1, the sensor 52 is a pressure sensor that measures (directly or indirectly) melt pressure of the molten thermoplastic material 24 in the nozzle 26. The sensor 52 generates an electrical signal that is transmitted to the controller 50. The controller 50 then commands the screw control 36 to advance the screw 22 at a rate that maintains a substantially constant melt pressure of the molten thermoplastic material 24 in the nozzle 26. While the sensor 52 may directly measure the melt pressure, the sensor 52 may measure other characteristics of the molten thermoplastic material 24, such as temperature, viscosity, flow rate, etc, that are indicative of melt pressure. Likewise, the sensor 52 need not be located directly in the nozzle 26, but rather the sensor 52 may be located at any location within the injection system 12 or mold 28 that is fluidly connected with the nozzle 26. If the sensor 52 is not located within the nozzle 26, appropriate correction factors may be applied to the measured characteristic to calculate the melt pressure in the nozzle 26. In yet other embodiments, the sensor 52 need not be fluidly connected with the nozzle. Rather, the sensor could measure clamping force generated by the clamping system 14 at a mold parting line between the first and second mold parts 25, 27. In one aspect, the controller may maintain the pressure according to the input from the sensor.

Although an active, closed loop controller 50 is illustrated in FIG. 1, other pressure regulating devices may be used instead of the closed loop controller 50. For example, a pressure regulating valve (not shown) or a pressure relief valve (not shown) may replace the controller 50 to regulate the melt pressure of the molten thermoplastic material 24. More specifically, the pressure regulating valve and pressure relief valve can prevent overpressurization of the mold 28. Another alternative mechanism for preventing overpressurization of the mold 28 is to activate an alarm when an overpressurization condition is detected.

Turning now to FIG. 2, an example molded part 100 is illustrated. The molded part 100 is a thin-walled part. Molded parts are generally considered to be thin-walled when a length of a flow channel L divided by a thickness of the flow channel T is greater than 100 (i.e., L/T>100). In some injection molding industries, thin-walled parts may be defined as parts having an L/T>200, or an L/T>250. The length of the flow channel L is measured from a gate 102 to a flow channel end 104. Thin-walled parts are especially prevalent in the consumer products industry.

Molded parts are generally considered to be thin-walled when a length of a flow channel L divided by a thickness of the flow channel T is greater than 100 (i.e., L/T>100). For mold cavities having a more complicated geometry, the L/T ratio may be calculated by integrating the T dimension over the length of the mold cavity 32 from a gate 102 to the end of the mold cavity 32, and determining the longest length of flow from the gate 102 to the end of the mold cavity 32. The L/T ratio can then be determined by dividing the longest length of flow by the average part thickness.

Thin-walled parts present certain obstacles in injection molding. For example, the thinness of the flow channel tends to cool the molten thermoplastic material before the material reaches the flow channel end 104. When this happens, the thermoplastic material freezes off and no longer flows, which results in an incomplete part. To overcome this problem, traditional injection molding machines inject the molten thermoplastic material at very high pressures, typically greater than 15,000 psi, so that the molten thermoplastic material rapidly fills the mold cavity before having a chance to cool and freeze off. This is one reason that manufacturers of the thermoplastic materials teach injecting at very high pressures. Another reason traditional injection molding machines inject at high pressures is the increased shear, which increases flow characteristics, as discussed above. These very high injection pressures require the use of very hard materials to form the mold 28 and the feed system.

Traditional injection molding machines use tool steels or other hard materials to make the mold. While these tool steels are robust enough to withstand the very high injection pressures, tool steels are relatively poor thermal conductors. As a result, very complex cooling systems are machined into the molds to enhance cooling times when the mold cavity is filled, which reduces cycle times and increases productivity of the mold. However, these very complex cooling systems add great time and expense to the mold making process.

The inventors have discovered that shear-thinning thermoplastics (even minimally shear-thinning thermoplastics) may be injected into the mold 28 at low, substantially constant, pressure without any significant adverse affects. A variety of thermoplastic materials can be used in the low, substantially constant pressure injection molding methods of the disclosure. In one embodiment, the molten thermoplastic material has a viscosity, as defined by the melt flow index of about 0.1 g/10 min to about 500 g/10 min, as measured by ASTM D1238 performed at temperature of about 230 C with a 2.16 kg weight. For example, for polypropylene the melt flow index can be in a range of about 0.5 g/10 min to about 200 g/10 min. Other suitable melt flow indexes include about 1 g/10 min to about 400 g/10 min, about 10 g/10 min to about 300 g/10 min, about 20 to about 200 g/10 min, about 30 g/10 min to about 100 g/10 min, about 50 g/10 min to about 75 g/10 min, about 0.1 g/10 min to about 1 g/10 min, or about 1 g/10 min to about 25 g/10 min. The MFI of the material is selected based on the application and use of the molded article. For examples, thermoplastic materials with an MFI of 0.1 g/10 min to about 5 g/10 min may be suitable for use as preforms for Injection Stretch Blow Molding (ISBM) applications. Thermoplastic materials with an MFI of 5 g/10 min to about 50 g/10 min may be suitable for use as caps and closures for packaging articles. Thermoplastic materials with an MFI of 50 g/10 min to about 150 g/10 min may be suitable for use in the manufacture of buckets or tubs. Thermoplastic materials with an MFI of 150 g/10 min to about 500 g/10 min may be suitable for molded articles that have extremely high L/T ratios such as a thin plate. Manufacturers of such thermoplastic materials generally teach that the materials should be injection molded using melt pressures in excess of 6000 psi, and often in great excess of 6000 psi. Contrary to conventional teachings regarding injection molding of such thermoplastic materials, embodiments of the low, constant injection molding method of the disclosure advantageously allow for forming quality injection molded parts using such thermoplastic materials and processing at melt pressures below 6000 psi, and possibly well below 6000 psi.

The thermoplastic material can be, for example, a polyolefin. Exemplary polyolefins include, but are not limited to, polypropylene, polyethylene, polymethylpentene, and polybutene-1. Any of the aforementioned polyolefins could be sourced from bio-based feedstocks, such as sugarcane or other agricultural products, to produce a bio-polypropylene or bio-polyethylene. Polyolefins advantageously demonstrate shear thinning when in a molten state. Shear thinning is a reduction in viscosity when the fluid is placed under compressive stress. Shear thinning can beneficially allow for the flow of the thermoplastic material to be maintained throughout the injection molding process. Without intending to be bound by theory, it is believed that the shear thinning properties of a thermoplastic material, and in particular polyolefins, results in less variation of the materials viscosity when the material is processed at low pressures. As a result, embodiments of the method of the disclosure can be less sensitive to variations in the thermoplastic material, for example, resulting from colorants and other additives as well as processing conditions. This decreased sensitivity to batch-to-batch variations of the properties thermoplastic material can also advantageously allow post-industrial and postconsumer recycled plastics to be processed using embodiments of the method of the disclosure. Post-industrial, post consumer recycled plastics are derived from end products that have completed their life cycle as a consumer item and would otherwise have been disposed of as a solid waste product. Such recycled plastic, and blends of thermoplastic materials, inherently have significant batch-to-batch variation of their material properties.

The thermoplastic material can also be, for example, a polyester. Exemplary polyesters include, but are not limited to, polyethylene terphthalate (PET). The PET polymer could be sourced from bio-based feedstocks, such as sugarcane or other agricultural products, to produce a partially or fully bio-PET polymer. Other suitable thermoplastic materials include copolymers of polypropylene and polyethylene, and polymers and copolymers of thermoplastic elastomers, polyester, polystyrene, polycarbonate, poly(acrylonitrile-butadiene-styrene), poly(lactic acid), bio-based polyesters such as poly(ethylene furanate) polyhydroxyalkanoate, poly(ethylene furanoate), (considered to be an alternative to, or drop-in replacement for, PET), polyhydroxyalkanoate, polyamides, polyacetals, ethylene-alpha olefin rubbers, and styrene-butadiene-styrene block copolymers. The thermoplastic material can also be a blend of multiple polymeric and non-polymeric materials. The thermoplastic material can be, for example, a blend of high, medium, and low molecular polymers yielding a multi-modal or bi-modal blend. The multi-modal material can be designed in a way that results in a thermoplastic material that has superior flow properties yet has satisfactory chemo/physical properties. The thermoplastic material can also be a blend of a polymer with one or more small molecule additives. The small molecule could be, for example, a siloxane or other lubricating molecule that, when added to the thermoplastic material, improves the flowability of the polymeric material.

Other additives may include inorganic fillers such calcium carbonate, calcium sulfate, talcs, clays (e.g., nanoclays), aluminum hydroxide, CaSiO3, glass formed into fibers or microspheres, crystalline silicas (e.g., quartz, novacite, crystallobite), magnesium hydroxide, mica, sodium sulfate, lithopone, magnesium carbonate, iron oxide; or, organic fillers such as rice husks, straw, hemp fiber, wood flour, or wood, bamboo or sugarcane fiber.

Other suitable thermoplastic materials include renewable polymers such as nonlimiting examples of polymers produced directly from organisms, such as polyhydroxyalkanoates (e.g., poly(beta-hydroxyalkanoate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate, NODAX (Registered Trademark)), and bacterial cellulose; polymers extracted from plants, agricultural and forest, and biomass, such as polysaccharides and derivatives thereof (e.g., gums, cellulose, cellulose esters, chitin, chitosan, starch, chemically modified starch, particles of cellulose acetate), proteins (e.g., zein, whey, gluten, collagen), lipids, lignins, and natural rubber; thermoplastic starch produced from starch or chemically starch and current polymers derived from naturally sourced monomers and derivatives, such as bio-polyethylene, bio-polypropylene, polytrimethylene terephthalate, polylactic acid, NYLON 11, alkyd resins, succinic acid-based polyesters, and bio-polyethylene terephthalate.

The suitable thermoplastic materials may include a blend or blends of different thermoplastic materials such in the examples cited above. As well the different materials may be a combination of materials derived from virgin bio-derived or petroleum-derived materials, or recycled materials of bio-derived or petroleum-derived materials. One or more of the thermoplastic materials in a blend may be biodegradable. And for non-blend thermoplastic materials that material may be biodegradable.

Parts molded at low, substantially constant, pressures exhibit some superior properties as compared to the same part molded at a conventional high pressure. This discovery directly contradicts conventional wisdom within the industry that teaches higher injection pressures are better. Without being bound by theory, it is believed that injecting the molten thermoplastic material into the mold 28 at low, substantially constant, pressures creates a continuous flow front of thermoplastic material that advances through the mold from a gate to a farthest part of the mold cavity. By maintaining a low level of shear, the thermoplastic material remains liquid and flowable at much lower temperatures and pressures than is otherwise believed to be possible in conventional high pressure injection molding systems.

Turning now to FIG. 3, a typical pressure-time curve for a conventional high pressure injection molding process is illustrated by the dashed line 200. By contrast, a pressure-time curve for the disclosed low constant pressure injection molding machine is illustrated by the solid line 210.

In the conventional case, melt pressure is rapidly increased to well over 15,000 psi and then held at a relatively high pressure, more than 15,000 psi, for a first period of time 220. The first period of time 220 is the fill time in which molten plastic material flows into the mold cavity. Thereafter, the melt pressure is decreased and held at a lower, but still relatively high pressure, 10,000 psi or more, for a second period of time 230. The second period of time 230 is a packing time in which the melt pressure is maintained to ensure that all gaps in the mold cavity are back filled. The mold cavity in a conventional high pressure injection molding system is packed from the end of the flow channel back to towards the gate. As a result, plastic in various stages of solidification are packed upon one another, which may cause inconsistencies in the finished product, as discussed above. Moreover, the conventional packing of plastic in various stages of solidification results in some non-ideal material properties, for example, molded-in stresses, sink, non-optimal optical properties, etc.

The constant low pressure injection molding system, on the other hand, injects the molten plastic material into the mold cavity at a substantially constant low pressure for a single time period 240. The injection pressure is typically less than 6,000 psi. By using a substantially constant low pressure, the molten thermoplastic material maintains a continuous melt front that advances through the flow channel from the gate towards the end of the flow channel. Thus, the plastic material remains relatively uniform at any point along the flow channel, which results in a more uniform and consistent finished product. By filling the mold with a relatively uniform plastic material, the finished molded parts form crystalline structures that have better mechanical and optical properties than conventionally molded parts. Moreover, the skin layers of parts molded at low constant pressures exhibit different characteristics than skin layers of conventionally molded parts. As a result, the skin layers of parts molded under low constant pressure can have better optical properties than skin layers of conventionally molded parts.

By maintaining a substantially constant and low (e.g., less than 6000 psi) melt pressure within the nozzle, more machineable materials may be used to form the mold 28 and/or feed system. For example, the mold 28 illustrated in FIG. 1 may be formed of a material having a milling machining index of greater than 100%, a drilling machining index of greater than 100%, a wire EDM machining index of greater than 100%, a graphite sinker EDM machining index of greater than 200%, or a copper sinker EDM machining index of greater than 150%. The machining indexes are based upon milling, drilling, wire EDM, and sinker EDM tests of various materials. The test methods for determining the machining indices are explained in more detail below. Examples of machining indexes for a sample of materials is compiled below in Table 1.

TABLE 1 Machining Technology Milling Drilling Wire EDM Sinker EDM-Graphite Sinker EDM-Copper Spindle Index Spindle Index Index Index Index Load % Load % time % time % time % Material 1117 0.72 100% 0.32 100% 9:34 100% 0:14:48 100% 0:24:00 100% steel* 6061 0.5 144% 0.2 160% 4:46 201% 0:05:58 248% 0:15:36 154% Aluminum alloy 7075 0.55 131% 0.24 133% 4:48 199% 0:05:20 278% 0:12:27 193% Aluminum alloy QC-10 a 0.56 129% 0.24 133% 4:47 200% 0:05:11 286% 0:12:21 194% Aluminum alloy 41-40 0.92  78% 0.37  86% 9:28 101% 0:09:36 154% 0:19:20 124% Steel 420 1.36  53% 0.39  82% 8:30 113% 0:10:12 145% 0:23:20 103% stainless steel A2 0.97  74% 0.45  71% 8:52 108% 0:08:00 185% 0:20:12 119% AISI tool steel S7 1.2  60% 0.43  74% 9:03 106% 0:12:53 115% 0:20:58 114% AISI tool steel P20 1.1  65% 0.38  84% 9:26 101% 0:11:47 126% 0:20:30 117% AISI mold steel PX5 1.12  64% 0.37  86% 9:22 102% 0:12:37 117% 0:23:18 103% AISI mold steel MOLDMAX b 0.8  90% 0.36  89% 6:00 159% 6:59:35  4% 1 0:43:38  55% 3 HH Beryllium copper mold alloy AMPCOLOY c 0.62 116% 0.32 100% 6:53 139% 3:13:41  8% 2 0:30:21  79% 4 944 steel alloy a-QC-10 available from Alcoa, Inc. of Pittsburgh, Pennsylvania, United States b-MOLDMAX HH available from Brush Wellman, Inc. of Mayfield Heights, Ohio, United States c-AMPCOLOY 944 available from Ampco Metal, SA of Marly, Switzerland *1117 is the benchmark material for this test. Published data references 1212 carbonsteel as the benchmark material. 1212 was not readily available. Of the published data, 1117 was the closest in composition and machining index percentage (91%). 1 Significant graphite electrode wear: ~20% 2 graphite electrode wear: ~15% 3 Cu electrode wear: ~15% 4 Cu electrode wear: ~3%

Using easily machineable materials to form the mold 28 results in greatly decreased manufacturing time and thus, a decrease in manufacturing costs. Moreover, these machineable materials generally have better thermal conductivity than tool steels, which increases cooling efficiency and decreases the need for complex cooling systems.

When forming the mold 28 of these easily machineable materials, it is also advantageous to select easily machineable materials having good thermal conductivity properties. Materials having thermal conductivities of more than 30 BTU/HR FT ° F. are particularly advantageous. For example easily machineable materials having good thermal conductivities include, but are not limited to, QC-10 (an aluminum alloy available from Alcoa, Inc. of Pittsburgh, Pa., United States), DURAMOLD-5 (an aluminum alloy available from Vista Metals Corp. of Fontana, California, United States), and HOKOTOL (an aluminum alloy available from Aleris International, Inc. of Beachwood, Ohio, United States). Materials with good thermal conductivity more efficiently transmit heat from the thermoplastic material out of the mold. As a result, more simple cooling systems may be used. Additionally, non-naturally balanced feed systems are also possible for use in the constant low pressure injection molding machines described herein.

One example of a multi-cavity mold 28 is illustrated in FIGS. 4A and 4B. Multi-cavity molds generally include a feed manifold 60 that directs molten thermoplastic material from the nozzle 26 to the individual mold cavities 32. The feed manifold 60 includes a sprue 62, which directs the molten thermoplastic material into one or more runners or feed channels 64. Each runner may feed multiple mold cavities 32. In many high capacity injection molding machines, the runners are heated to enhance flowability of the molten thermoplastic material. Because viscosity of the molten thermoplastic material is very sensitive to shear and pressure variations at high pressures (e.g., above 10,000 psi), conventional feed manifolds are naturally balanced to maintain uniform viscosity. Naturally balanced feed manifolds are manifolds in which molten thermoplastic material travels an equal distance from the sprue to any mold cavity. Moreover, the cross-sectional shapes of each flow channel are identical, the number and type of turns are identical, and the temperatures of each flow channel are identical. Naturally balanced feed manifolds allow the mold cavities to be filled simultaneously so that each molded part has identical processing conditions and material properties.

FIG. 5 illustrates an example of a naturally balanced feed manifold 60. The naturally balanced feed manifold 60 includes a first flow path 70 from the sprue 62 to a first junction 72 where the first flow path 70 splits into second and third flow paths 74, 76, the second flow path terminating at a second gate 78 a and the third flow path 76 terminating at a third gate 78 b each gate serving an individual mold cavity (not shown in FIG. 5). Molten thermoplastic material flowing from the sprue 62 to either the second gate 78 a or the third gate 78 b travels the same distance, experiences the same temperatures, and is subjected to the same cross-sectional flow areas. As a result, each mold cavity is filled simultaneously with molten thermoplastic material having identical physical properties.

FIGS. 6A and 6B illustrate the naturally balanced manifold 60 schematically. The naturally balanced manifold 60 of FIGS. 6A and 6B is a multi-tier manifold. Each flow path 74, 76 has identical characteristics at identical locations along the flow path. For example, after the junction 72, each flow path narrows at the same distance. Moreover, each flow path serves an identical number of mold cavities 32. Naturally balanced flow manifolds 60 are critical to high pressure injection molding machines to maintain identical plastic flow properties and to ensure uniform parts.

FIGS. 7A and 7B illustrate another naturally balanced manifold 60. The naturally balanced manifold 60 of FIGS. 7A and 7B is a single tier manifold.

By contrast, FIGS. 8, 9A, and 9B illustrate non-naturally balanced manifolds with FIG. 8 illustrating an artificially balanced manifold and FIGS. 9A and 9B illustrating non-balanced manifolds.

The low constant pressure injection molding machine disclosed herein allows artificially balanced manifolds, and even unbalanced manifolds, to be used because thermoplastic materials injected at low constant pressure are not as sensitive to pressure differences or shear differences due to flow channel characteristic differences. In other words, the thermoplastic materials injected at low constant pressure retain more uniform and balanced material and flow properties regardless of differences in flow channel length, cross-sectional area, or temperature. This provides for substantially more balanced flow in naturally unbalanced designs, and, in cases where flow might be highly imbalanced, provides more uniform material properties throughout every part cavity.

The artificially balanced manifold 160 of FIG. 8 includes a sprue 62, a first flow channel 174, and a second flow channel 176. The first flow channel 174 terminates at a first gate 178 a and the second flow channel 176 terminates at a second gate 178 b. The first flow channel 174 is shorter than the second flow channel 176 in this embodiment. The artificially balanced manifold 160 varies some other parameter of the flow channel (e.g., cross-sectional area or temperature) so that the material flowing through the manifold 160 provides balanced flow to each cavity, similar to a naturally balanced manifold. In other words, thermoplastic material flowing through the first flow channel 174 will have about equal melt pressure to thermoplastic material flowing through the second flow channel 176. Because artificially balanced, or unbalanced, feed manifolds can include flow channels of different lengths, an artificially balanced, or unbalanced, feed manifold can make much more efficient use of space. Moreover, the feed channels and corresponding heater band channels can be machined more efficiently. Furthermore, naturally balanced feed manifolds are limited to molds having distinct, even numbers of mold cavities (e.g., 2, 4, 8, 16, 32, etc.). Artificially balanced, and unbalanced, feed manifolds may be designed to deliver molten thermoplastic material to any number of mold cavities.

The artificially balanced feed manifold 160 may also be constructed of a material having high thermal conductivity to enhance heat transfer to the molten thermoplastic material in hot runners, thus enhancing flow of the thermoplastic material. More specifically, the artificially balanced feed manifold 160 may be constructed of the same material as the mold to further reduce material costs and enhance heat transfer within the entire system.

FIGS. 9A and 9B illustrate non-balanced manifolds 260. The non-balanced manifolds 260 may include an odd number of mold cavities 232, and/or flow channels having different cross-sectional shapes, different number and type of turns, and/or the different temperatures. Moreover, the non-balanced manifolds 260 may feed mold cavities having different sizes, and or shapes, as illustrated in FIG. 9B, or mold cavities that are oriented differently from one another in a common face of the mold. Furthermore, the non-balanced manifolds 260 may feed an injection mold having more than 8 mold cavities and/or an injection mold that includes a guided ejection system.

In yet other embodiments, the artificially balanced manifolds and/or unbalanced manifolds may be used in molds having mold cavities in separate layers, such as a stack mold configuration, the manifolds feeding mold cavities in one, two, or more layers of the stack mold. Mold cavities may receive molten plastic from more than one gate or each individual gate could deliver more than one material sequentially to an individual mold cavity. Furthermore, more than one manifold may feed a cavity position that rotates within the mold from a first position, where a first material is introduced, to a second position, where a second material is introduced.

Furthermore, the non-naturally balanced feed systems described above may include one or more heating elements in thermal communication with one of the feed channels. The heating elements may be contained in a material having a thermal conductivity that is substantially the same as the material forming a majority of the mold or in a material having a thermal conductivity that is substantially the same as a material forming the feed system.

Drilling and Milling Machineability Index Test Methods

The drilling and milling machineability indices listed above in Table 1 were determined by testing the representative materials in carefully controlled test methods, which are described below.

The machineability index for each material was determined by measuring the spindle load needed to drill or mill a piece of the material with all other machine conditions (e.g., stock feed rate, spindle rpm, etc.) being held constant between the various materials. Spindle load is reported as a ratio of the measured spindle load to the maximum spindle torque load of 75 ft-lb @ 1400 rpm for the drilling or milling device. The index percentage was calculated as a ratio between the spindle load for 1117 steel to the spindle load for the test material. The test milling or drilling machine was a Hass VF-3 Machining Center.

Drilling Conditions

TABLE 2 Spot Drill 120 degree 0.5″ diameter, drilled to 0.0693″ depth Drill Bit 15/32″ diameter high speed steel uncoated jobber length bit Spindle Speed 1200 rpm Depth of Drill 0.5″ Drill Rate 3 in/min Other No chip break routine used

Milling Conditions

TABLE 3 Mill 0.5″ diameter 4 flute carbide flat bottom end mill, uncoated (SGS part # 36432 www.sgstool.com Spindle Speed 1200 rpm Depth of Cut 0.5″ Stock Feed Rate 20 in/min

For all tests “flood blast” cooling was used. The coolant was Koolrite 2290.

EDM Machineability Index Test Methods

The graphite and copper sinker EDM machineability indices listed above in Table 1 were determined by testing the representative materials in a carefully controlled test method, which is described below.

The EDM machineability index for the various materials were determined by measuring the time to burn an area (specifics below) into the various test metals. The machineability index percentage was calculated as the ratio of the time to burn into 1117 steel to time required to burn the same area into the other test materials.

Wire EDM

TABLE 4 Equipment Fanuc OB Wire 0.25 mm diameter hard brass Cut 1″ thick × 1″ length (1 sq.″) Parameters Used Fanuc on board artificial intelligence, override @ 100%

Sinker EDM—Graphite

TABLE 5 Equipment Ingersoll Gantry 800 with Mitsubishi EX Controller Wire System 3R pre-mounted 25 mm diameter Poco EDM 3 graphite Cut 0.1″ Z axis plunge Parameters Used Mitsubishi CNC controls with FAP EX Series Technology

Sinker EDM—Copper

TABLE 6 Equipment Ingersoll Gantry 800 with Mitsubishi EX Controller Wire System 3R pre-mounted 25 mm diameter Tellurium Copper Cut 0.1″ Z axis plunge Parameters Used Mitsubishi CNC controls with FAP EX Series Technology

The disclosed low constant pressure injection molding machines advantageously employ molds constructed from easily machineable materials. As a result, the disclosed low constant pressure injection molding machines are less expensive and faster to produce. Additionally, the disclosed low constant pressure injection molding machines are capable of employing more flexible support structures and more adaptable delivery structures, such as wider platen widths, increased tie bar spacing, elimination of tie bars, lighter weight construction to facilitate faster movements, and non-naturally balanced feed systems. Thus, the disclosed low constant pressure injection molding machines may be modified to fit delivery needs and are more easily customizable for particular molded parts.

It is noted that the terms “substantially,” “about,” and “approximately,” unless otherwise specified, may be utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. These terms are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. Unless otherwise defined herein, the terms “substantially,” “about,” and “approximately” mean the quantitative comparison, value, measurement, or other representation may fall within 20% of the stated reference.

It should now be apparent that the various embodiments of the products illustrated and described herein may be produced by a low constant pressure injection molding process. While particular reference has been made herein to products for containing consumer goods or consumer goods products themselves, it should be apparent that the low constant pressure injection molding method discussed herein may be suitable for use in conjunction with products for use in the consumer goods industry, the food service industry, the transportation industry, the medical industry, the toy industry, and the like. Moreover, one skilled in the art will recognize the teachings disclosed herein may be used in the construction of stack molds, multiple material molds including rotational and core back molds, in combination with in-mold decoration, insert molding, in mold assembly, and the like. Moreover, one skilled in the art will recognize the teachings disclosed herein may be used in the construction of stack molds, multiple material molds including rotational and core back molds, in combination with in-mold decoration, insert molding, in mold assembly, and the like.

Virtual modeling programs, such as Sigmasoft and Moldflow, can be used to predict pressure, filling rates, and cooling times needed to fill a mold cavity. These programs are capable of modeling processes controlled by polymer flow rate, pressures, or combinations of flow rate and pressure. These programs are used in the design of runners, gate location, and mold design.

All documents cited in the Detailed Description of the Invention are, in relevant part, incorporated herein by reference; the citation of any document is not to be construed as an admission that it is prior art with respect to the present invention. To the extent that any meaning or definition of a term in this written document conflicts with any meaning or definition of the term in a document incorporated by reference, the meaning or definition assigned to the term in this written document shall govern.

While particular embodiments have been illustrated and described herein, it should be understood that various other changes and modifications may be made without departing from the spirit and scope of the claimed subject matter. Moreover, although various aspects of the claimed subject matter have been described herein, such aspects need not be utilized in combination. It is therefore intended that the appended claims cover all such changes and modifications that are within the scope of the claimed subject matter. 

What is claimed is:
 1. An unbalanced feed system for an injection mold of an injection molding apparatus, the injection mold having a plurality of mold cavities, the plurality of mold cavities receiving molten plastic at a substantially constant viscosity, the unbalanced feed system comprising: wherein the feed system maintains a balance of 90% or greater; a hot runner having a first feed channel that terminates at a first mold cavity in the plurality of mold cavities and a second feed channel that terminates at a second mold cavity in the plurality of mold cavities, wherein the first feed channel and the second feed channel are unbalanced in their design due to a circular cross-sectional shape of the first feed channel varying along a length of the first feed channel and a non-circular cross-sectional shape of the second feed channel varying along a length of the second feed channel and the circular cross-sectional shape varying differently than the non-circular cross-sectional shape; a gate for receiving molten plastic, the gate being in fluid communication with one of the first and second feed channels; and a controller that maintains the molten plastic at a substantially constant melt pressure.
 2. The unbalanced feed system of claim 1 wherein the feed system maintains a balance of 95% or greater.
 3. The injection molding apparatus of claim 1 that injects the molten plastic into the mold cavities such that part weights for all of the mold cavities vary by less than 5%.
 4. The injection molding apparatus of claim 1 that injects the molten plastic into the mold cavities such that part weights for all of the mold cavities vary by less than 1%. 